1. Field of the Invention
This invention relates generally to controlling the drive train of an electric vehicle.
2. Description of the Prior Art
To increase the range of an electric vehicle, it is particularly important for the drive train to work as efficiently as possible. In a conventional drive train, the traction or electric motor and the transmission are designed such that both the requirements for performance of the electric vehicle and the requirements for efficient energy utilization are fulfilled.
Whereas the performance criteria, i.e. the criteria regarding performance of the drive train, in an exclusively electrically driven motor vehicle require high torque and power values, the typical drive cycle of an electric vehicle takes place in the low torque and low rotation speed range, since in typical urban drive cycles and homologation cycles, the electric motor is usually operated with a comparatively low power and torque profile.
In this low load range, however, a conventional electric motor has low efficiency. To achieve higher efficiencies of the drive train in a typical urban drive cycle and in typical homologation cycles, the design of the electric motor must have nominal values for power and torque corresponding to the respective load cycle. However in these cases, the performance criteria are not fulfilled as these performance criteria require higher nominal values for torque and power.
Designing the electric motor to have the optimum operating or working point in the homologation cycle load path, therefore, leads to a lower starting torque at high engine speeds, which in turn has the consequence that the electric motor is no longer able to deliver sufficiently high torque on a hill start or in scenarios with high rotation speed.
JP 2005161961 A discloses, in a hybrid electric vehicle, by means of an electronic hybrid control unit, increasing the torque distributed to the rear axle relative to the torque distributed to the front axle, in particular on occurrence of irregularities in operation of the electronic brake control, in order to further guarantee a functioning all-wheel drive.
US 2006/0151220 A1 discloses a vehicle with individually electrically driven wheels, wherein the rotation axes of the rotating electric machines used are offset relative to those of the individually electrically driven wheels in order to achieve an increase in drive power without increasing the radial or axial length of the machines.
U.S. Pat. No. 6,549,840 B1 discloses a vehicle control unit to control a motor vehicle with all-wheel drive, by means of which a front drive power to drive the pair of front wheels and a rear drive power to drive the pair of rear wheels are controlled as a function of a static and a dynamic vehicle state such that the sum of the front drive power and the rear drive power is equal to the value of the vehicle drive power desired by the driver.